The present invention relates to a driving technology for piezoelectric transformers that transform a voltage from a direct current voltage supply into a predetermined voltage using a piezoelectric effect, and more particularly, to a driving method and a driving circuit of piezoelectric transformers for constituting a power supply circuit for effectively turning on a discharge tube using the piezoelectric transformers.
Conventionally, as a power supply circuit for turning on a discharge tube, a driving circuit using a piezoelectric transformer was proposed. As a technology for driving such a piezoelectric transformer, a driving circuit was proposed, in which the piezoelectric transformer was driven by a sinusoidal wave of a transmission wave and an output of this piezoelectric transformer was input from one side of a load. See for example JP-A-275553/1996. In this driving circuit, as shown in FIG. 4, secondary sides of two autotransformers 33 and 34 are connected to two primary electrodes of the piezoelectric transformer 31, and primary sides of the autotransformers 33 and 34 are connected to a power voltage Vdd. Further, switching transistors 37 and 38 are connected to intermediate terminals of the autotransformers 33 and 34, a current flowing in a load 32 is detected and a drive frequency of the piezoelectric transformer 31 is determined by a frequency control circuit 35, and the drive frequency is input to a two phase driving circuit 36. By an output from this two phase driving circuit 36, the switching transistors 37 and 38 are alternately switched at a resonance frequency of the piezoelectric transformer 31. Primary and secondary inductances of these autotransformers 37 and 38 and an equivalent input capacity of the piezoelectric transformer 31 are set to construct a resonance circuit. In the circuit, two phases half wave sinusoidal waves shown in the figure are generated, and by alternately applying these two half wave sinusoidal waves to two primary side electrodes 31a and 31b of the piezoelectric transformer 31, a sinusoidal wave is applied to the primary side electrodes of the piezoelectric transformer 31. In this manner, the piezoelectric transformer 31 can apply a boosted output voltage to the load 32. Also, as other prior arts, a technology is shown, in which a half wave sinusoidal wave in JP-A-33349/1996 or a rectangular wave in JP-A-47265/1996 is input to a primary side of a piezoelectric transformer and an output therefrom is input to one side of a load.
On the other hand, as a driving circuit for turning on a discharge tube such as a cold cathode fluoresce lamp using an electromagnetic transformer, and further reducing luminance non-uniformity of the discharge tube, the technology thereof is disclosed in JP-A-20783/1994. In this circuit, as shown in FIG. 5, a periodicity signal such as a horizontal synchronizing signal of a video signal is input to a current control circuit 42 and a pulse direction control circuit 45 from a terminal 41. Also, an output from the current control circuit 42 is input to a gate of a switching element 43. A source side of this switching element 43 is grounded, and a drain side is connected to one end of a first primary winding 44a and the other end of a second primary winding 44b of a transformer 44. Outputs of the pulse direction control circuit 45 are a switching signal Q that is reversed every predetermined period of the horizontal synchronizing signal, and a signal Qbar with a phase opposite to that of the signal Q, and Q and Qbar are input to a base of a transistor 46a and a base of a transistor 46b, respectively. Collectors of the transistors 46a and 46b are connected to a power supply, and emitters thereof are connected to the other end of the first primary winding 44a and one end of the second primary winding 44b, respectively. One end of a secondary winding 44c of the transformer 44 is connected to one electrode of a discharge tube 47 and the other end of the secondary winding 44c is connected to the other electrode of the discharge tube 47.
In this circuit arrangement, if a signal as shown in FIG. 6A is input from the terminal 41, a signal as shown in FIG. 6B is supplied to the gate of the switching element 43 from the current control circuit 42, and switching signals as shown in FIGS. 6C and 6D are formed from the pulse direction control circuit 45, which are reversed at an intermediate position of the signal of FIG. 6A. By supplying the signal of FIG. 6C to the base of the transistor 46a and supplying the signal of FIG. 6D to the base of the transistor 46b, respectively, a current alternately flows in the first primary winding 44a and the second primary winding 44b. Since the first primary winding 44a and the second primary winding 44b are wound in a direction opposite to each other, a pulse voltage that is reversed every predetermined period as shown in FIG. 6E occurs between one end and the other end of the secondary winding 44c of the transformer 44, and this voltage is applied to one side and the other side of the discharge tube 47.
In addition to this, as a prior art for reducing luminance non-uniformity of a backlight of a liquid crystal panel, a technology thereof is disclosed in JP-A-27918/1990. FIG. 7 is a circuit diagram thereof, and on a reverse surface of the liquid crystal panel, the number M (M is an integer of one or more than one) of discharge tubes TM are arranged in a direction extending along a line, and a converter 51, a low frequency pulse generator 52 and a tube control signal generator 53 are provided in each of the discharge tubes. In an arrangement of the circuit, the converter 51 is connected to the discharge tube T1, and a DC voltage VO is input so as to be able to apply a rectangular wave of a high frequency voltage with a period THF as shown in FIG. 8C for example to the discharge tube T1. A low frequency control signal VBF as shown in FIG. 8B is input to the converter 51, which creates a change of the luminance of the discharge tube T1. The low frequency control signal VBF is a low frequency period pulse having a chop speed TBF/Tm that is adjustable and modulates a high frequency voltage VHF applied to the discharge tube T1. Also, the low frequency control signal VBF is generated by the low frequency pulse generator 52 based on a control signal ST input from the tube control signal generator 53. And, the signal ST is input to a first input of a comparator 55 through an integrator 54. An output signal VS from a slope generator 56, as shown in FIG. 8A, is input to a second input of the comparator 55. An output voltage Vm of the integrator 54 is compared with the slope signal VS, and the low frequency control signal VBF as shown in FIG. 8B is generated. When the voltage ST changes, the voltage Vm changed in accordance with-the change, and a pulse width Tm of the low frequency rectangular wave signal VBF changes. By means of this signal VBF, the voltage VHF applied to the discharge tube T1 is controlled. Through this control, luminance LT of the discharge tube T1 is continuously modulated as shown in FIG. 8D. Also, average luminance of the discharge tube T1 is presented as LTm of FIG. 8D. FIG. 9A to FIG. 9D indicate the same signals as in FIG. 8A to FIG. 8D in case that the control signal ST changes. In FIG. 9, by changing a value of a modulation time Tm, duration of the voltage VHF applied to the discharge tube T1 is changed. This control is conducted for the number M of discharge tubes TM arranged in a direction extending along a line on the reverse surface of the liquid crystal panel, and luminance non-uniformity of the entire liquid crystal panel is reduced.
The first task in the above prior arts is that in case the load is a discharge tube and a piezoelectric transformer is driven as disclosed in the above-described JP-A-275553/1996, JP-A-33349/1996 and JP-A-47285/1996, a high voltage is applied to only one side of the discharge tube and the discharge tube is turned on, luminance of a low voltage side of the discharge tube to which the high voltage is not applied is low and luminance non-uniformity between a high voltage side and the low voltage side of the discharge tube occurs. This phenomenon occurs like in an electromagnetic transformer and occurs even though the discharge tube is turned on like in the manner as disclosed in JP-A-20783/1994. Also, like in JP-A-27916/1990 for example, this phenomenon occurs in case that a high voltage is input to one side of a discharge tube used in a liquid crystal panel, and luminance of the liquid crystal panel is controlled to be constant by changing an application period of time when the high voltage is applied to a predetermined number of discharge tubes. The reason thereof is that due to a floating capacitor formed between the discharge tube and a reflection plate of the discharge tube, a current flowing in the discharge tube flows into the reflection plate. A structure of a backlight in which a discharge tube is placed right below is shown in FIG. 10A. Therefore, as shown in FIG. 10B, a floating capacitor is formed between the discharge tube and the reflection plate of the discharge tube, and a current becomes to flow into the reflection plate from the discharge tube. Accordingly, a current flowing on a low voltage side of the discharge tube is smaller compared with that on a high voltage side, and luminance on the low voltage side also becomes to be lower.
Also, the second task is that in the case a high voltage is input from one side of the discharge tube, it is impossible to effectively turn on the discharge tube. The reason thereof is that as a voltage applied to the discharge tube is higher, a current flowing into the floating capacitor formed between the discharge tube and the reflection plate is more. Assuming that a capacity value of the entire floating capacitor is C, a voltage applied to the discharge tube is V, and an energy for charging and discharging the floating capacitor by means of a flow of a current into the floating capacitor is W, a relation such as an equation (1) is established. EQU W=(1/2).times.CxV.sup.2 (1)
As shown in the equation (1), the energy W for charging and discharging the floating capacitor is proportional to the square of the voltage applied to the discharge tube, and also, since, when the voltage applied to the discharge tube is higher, a reactive power for charging and discharging this floating capacitor increases due to a resistance component of the discharge tube, loss of this power on an output side also increases.
The third task is that although, in case of using a piezoelectric transformer in place of the electromagnetic transformer, two piezoelectric transformers are necessary for inputting high voltages from either side of the discharge tube, it is sometimes impossible to obtain the same output voltages when the two piezoelectric transformers are driven at the same drive frequencies. The reason thereof is as follows: The drive frequencies of the two piezoelectric transformers which are input from the either side of the discharge tube are the same as each other. However, if dispersion exists in each drive frequency of the piezoelectric transformers T1 and T2 and ratios of rise thereof as shown in FIG. 11, that is, generally, since the piezoelectric transformers have high Q, if dispersion of a resonance frequency exists, even the slight dispersion gives the ratios of rise a large difference, and thereby, the drive voltages of the discharge tube becomes to be unbalanced.